The present application relates generally to electro-migration stress testing mechanisms and more specifically to mechanisms for performing electro-migration stress testing using a parallel array architecture.
Electro-migration is the transportation of material caused by the gradual movement of ions in a conductor. Specifically, electro-migration is the phenomenon in which the metal ions of a metal conductor will tend to migrate in the opposite direction of the flow of current through the metal conductor. The ions in the conductor move due to the momentum of transfer between conducting electrons and diffusing metal atoms.
FIG. 1 illustrates an example of how metal ions 10 can migrate over time along a conductor 12, even migrating beyond the boundary of the conductor 12 itself. As discussed hereafter, factors that contribute to electro-migration include the cross-sectional area of the conductor 12, current density through the conductor 12, and temperature. Over the course of time, the metal ions 10 of the conductor 12 can migrate to such a degree that voids 14 are created where the ions have left. These voids 14, when large enough, can impede the flow of current and therefore present a noticeable increase in resistance of the conductor 12.
In an extreme condition, electro-migration can create voids that span across the width of the conductor 12, thereby electrically isolating one portion of the conductor 12 from another and resulting in an open circuit condition. The open circuit condition results in a failure of the conductor and any interconnects that utilize this conductor. Such electro-migration effects are important in applications where high direct current densities are used, such as in microelectronics. As the size of the microelectronics decrease, the practical significance of electro-migration increases.
Electro-migration decreases the reliability of such microelectronics and thus, the resulting integrated circuits that utilize these microelectronics. In worst cases, electro-migration leads to the eventual loss of one or more connections and intermittent failure of the entire circuit. This can be catastrophic in microelectronics used in space, military, and safety electronics, and is generally troublesome and costly in commercial electronics.
Due to the relatively long life span of microelectronic interconnects, and the short product life cycle of most consumer integrated circuits, it is not practical to characterize a product's electro-migration under real operating conditions. To the contrary, Black's equation is commonly used to predict the life span of microelectronic interconnects in integrated circuits tested under “stress.” Black's equation is as follows:Mean Time to Failure (MTTF)=A(J−n)eEα/kT where A is a constant based on the cross-sectional area of the interconnect, J is the current density, Eα is the activation energy, k is the Boltzmann's constant, T is the temperature, and n is the scaling factor (usually set to 2 according to Black's model). As can be seen from this equation, the current density J and the temperature T are the dominant factors in the design process that affect electro-migration. The temperature T factor appears in the exponent of the equation and thus, it strongly affects the mean time to failure (MTTF) of the interconnect. Therefore, in order for an interconnect to remain reliable in rising temperatures, the maximum tolerable current density of the conductor must decrease.
Electro-migration effects occur over a long period of time, e.g., many years, and thus, as mentioned above, it is not feasible to perform electro-migration testing of active microelectronic device under normal operating conditions. However, it is important to characterize such electro-migration effects with independent control over the current density and temperature in order to determine the reliability of the interconnects of the microelectronics, especially in such technology areas as space, military, and safety electronics. As a result, electro-migration testing typically involves applying a “stress” to a small number of devices under test (DUTs), in order to accelerate the aging of the DUTs to determine the Mean Time to Failure. The “stress” of such stress testing is generated by applying external heat to the entire wafer on which the DUTs reside and providing current to each DUT to effectively age the DUT to determine the temperature and current density at which the DUT fails. For example, the wafer is placed on or in close proximity to a tester heat chuck which increases the temperature of the DUT to between 300 and 400 degrees Celsius. Four probes are typically used to measure the electro-migration of the DUT, 2 probes to force a current through the DUT and 2 probes to sense the voltage across the DUT. These probes must typically be placed on the pins and left there for long periods of time (sometimes around 100 hours), in order to perform the electro-migration measurements.
The results of applying the stress to the DUT is observed to determine how the DUT's characteristics change during the time of the test. This information may then be used to predict how the conductors of the actual microelectronic component might change during the normal life of the component. During this test, a sensor detects any changes in resistance of the DUT as a result of the application of the high current and temperature. A noticeable change in resistance signifies a potential electro-migration problem.
A severe limitation to this characterization strategy is the bottleneck in DUT throughput. Only a relatively small number of devices can be characterized at a time due to the limited number of tester channels available. Since each DUT requires 4 dedicated tester channels, it lacks the scalability necessary for massive DUT parallelization that is required for statistical studies of electro migration. One technique to increase DUT parallelization without increasing tester requirements is to use active devices such as NFETs and PFETs; however the excessive temperature environment established when heating the entire wafer during stress prohibits the use of active devices.